Thermoplastic resin microporous membranes are used widely as filters and separators.
Specifically, they are used in battery separators for lithium ion rechargeable batteries, nickel-metal hydride batteries, nickel-cadmium batteries, or polymer batteries, separators for electric double layer capacitors, various filters such as ultrafiltration membranes, microfiltration membranes, and the like, moisture permeation waterproof clothes, medical materials and the like.
In particular, when used as a separator for a lithium ion rechargeable battery, with the increasing capacity and power and decreasing weight of batteries, the thermoplastic resin microporous membrane requires mechanical characteristics as well as physical properties such as heat resistance, permeability, dimensional stability, pore blocking characteristics, membrane melt-puncture characteristics, electrical insulating properties, electrolytic solution resistance, oxidation resistance, and the like.
Additionally, polyolefin porous membranes have been advantageously used as separators for lithium ion rechargeable batteries due to having ion permeability due to electrolytic solution impregnation and possessing not only excellent electrical insulating properties, electrolytic solution resistance, and anti-oxidation properties, but also a pore blocking effect, which blocks the electrical current to prevent excessive temperature increases in the temperature range of about 120 to 150° C. when battery temperature increases abnormally.
On the other hand, if the temperature continues to increase even after pore blocking, the membrane may puncture due to a decrease in viscosity of the polyethylene that constitutes the membrane and shrinkage of the membrane.
Furthermore, polyolefin porous membranes further require improved adhesion to electrode materials for improvement in cycle characteristics of batteries and improved wettability toward electrolytic solution for productivity improvement.
To solve these problems, lamination of various modified porous layers on a porous membrane has been studied.
As modified porous layers, polyamideimide resin, polyimide resin, and polyamide resin, which have both good heat resistance and good wettability toward electrolytic solution, fluorine-based resin, which exhibits good adhesion toward electrode, and the like are preferably used.
A modified porous layer refers to a layer that includes a resin that provides or improves at least one of the functions among heat resistance, adhesion to an electrode material, wettability toward electrolytic solution, and the like.
Furthermore, it is necessary to increase the area not only of the electrode but also of the separator to fill a container in order to increase battery capacity, and a decrease in thickness of the separator is expected.
However, since the thinner porous membrane may experience deformation in its planar directions, a modified porous layer laminated on a thin porous membrane in a battery separator may delaminate during processing, slit processing, or the battery assembly process, and the safety may be compromised.
Additionally, it is anticipated that the battery assembly process will be sped up for the purpose of reducing cost.
To obtain a separator that has few problems, such as delamination of the modified porous layer even in high-speed processing, a polyolefin porous membrane having high adhesion with a modified porous layer that can withstand high-speed processing is required.
However, when the resin included in the modified porous layer is sufficiently permeated into the polyolefin porous membrane in order to improve the adhesion, large increases in air permeation resistance have been problematic.
Patent Document 1 discloses that a solution containing polyvinylidene fluoride is coated on one side of a 9 μm thick polyethylene porous membrane, and due to a fraction of polyvinylidene fluoride resin adequately penetrating into fine pores of the polyethylene porous membrane, it exhibits an anchor effect. As a result, a composite porous membrane with a peel strength (T-peel strength) of 1.0 to 5.3 N/25 mm at the interface between the polyethylene porous membrane and the polyvinylidene fluoride coating layer is obtained.
Patent Document 2 discloses that a heat-resistant porous layer, which contains acrylic resin, an N-vinyl acetamide polymer or a thickener of water soluble cellulose derivative, and plate-like boehmite, is disposed on a 16 μm thick, corona-discharge treated polyethylene porous membrane, resulting in a separator with a 180° peel strength (T-peel strength) of 1.1 to 3.0 N/10 mm at the interface between the polyethylene porous membrane and the heat-resistant porous layer.
Patent Document 3 discloses a method of producing a porous membrane, in which a polyethylene solution that comprises 30 parts by weight of a polyethylene composition (comprising 20 wt. % of ultrahigh molecular weight polyethylene (UHMWPE) with a weight average molecular weight of 2.0×106, 80 wt. % of high density polyethylene (HDPE) with a weight average molecular weight of 3.5×105, and antioxidant) and 70 parts by weight of liquid paraffin are extruded from an extruder at 190° C., and the extrudate is drawn by a chill-roll kept at 50° C. The resultant gel-like product is stretched biaxially to 500%×500%, and a porous membrane is obtained.
In Patent Document 4, the method of producing a microporous membrane is disclosed, in which polyethylene solution similar to that in Patent Document 3 is extruded from an extruder, and the extrudate is drawn by a chill-roll kept at 0° C. The resultant gel-like product is stretched biaxially to 500%×500%, and the microporous membrane is obtained.
In Patent Document 5, Working Example 1 discloses that a polyethylene solution that comprises 50 parts by mass of a composition (comprising 47.5 parts by mass of polyethylene with a viscosity-average molecular weight of 200,000, 2.5 parts by mass of polypropylene with a viscosity-average molecular weight of 400,000, and antioxidant) and 50 parts by mass of liquid paraffin is extruded from an extruder at 200° C., and the extrudate is drawn by a chill-roll kept at 25° C. to obtain a gel-like product, which is then biaxially stretched to 700%×640%, and a polyolefin resin porous membrane is obtained. A coating layer comprising polyvinyl alcohol and alumina particles is laminated on one side of the obtained polyolefin resin porous membrane, to yield a multi-layer porous membrane.
In Patent Document 6, Working Example 6 discloses that a polyethylene solution that comprises 30 wt. % of a polyethylene composition (comprising polyethylene of weight average molecular weight 4,150,000 and polyethylene of weight average molecular weight 560,000 in a weight ratio of 1:9) and 70 wt. % of solvent mix of liquid paraffin and decalin is extruded from an extruder at 148° C. and the extrudate is cooled in a water bath. The resultant gel-like product is then stretched biaxially to 550%×1,100%, and a polyethylene porous membrane is obtained. A coating layer comprising meta-type wholly aromatic polyamide and alumina particles is laminated on both sides of the obtained polyethylene resin porous membrane, to yield a non-water-based separator for a rechargeable battery.
Patent Document 7 discloses a polyolefin microporous membrane with an embossed pattern of diagonal lattices obtained by passing a gel sheet between an embossing roll and a back-up roll before stretching.
However, the separators described in Patent Documents 1 to 7 will not sufficiently ensure safety due to partial delamination of the modified porous layers during slit processing or battery assembly process, when it is required that the manufacturing processes are sped up and thinner separators are provided accompanying the lower cost and higher capacity in the near future.
In particular, as the polyolefin resin porous membrane that serves as a substrate becomes thinner, it becomes more difficult to ensure safety sufficiently since it is more difficult to achieve enough anchoring of modified porous layers to the polyolefin resin porous membrane.